In 1936, Keynes published The General Theory of Employment, Interest and Money, usually regarded as the 20th century's most important work on economics. Central to the General Theory's larger argument was "the principle of effective demand", that employment depended on the demand for output. With demand or "expenditure" therefore being so important, economists needed a view on the determinants of national expenditure.
Despite the single theory implied by book's title, it actually contained two theories about aggregate expenditure. The first was traditional, that expenditure depended on the quantity of money. The second was novel, that expenditure could be equated with the sum of consumption and investment and should be seen as a multiple of investment. The revolutionary element in the General Theory arose from the sidelining of the monetary side of the story and the promotion of the new, alternative theory.
The technical claim that expenditure was a multiple of investment led to the more politically charged statement that expenditure was a multiple of private and public investment. Further, if private investment were too weak to motivate a level of aggregate expenditure consistent with high employment, the state should deliberately boost public investment. So Keynes proceeded to assert that "a somewhat comprehensive socialisation of investment will prove the only means of securing an approximation to full employment".
So it is the second of the two theories — the multiplier theory of aggregate demand — that culminates in the set of notions widely seen nowadays as distinctively "Keynesian". This goes much beyond Keynes in stressing the virtues of fiscal policy and the vices of monetary policy. Some of them also boast that the relatively full employment in most industrial countries between 1945-1975 resulted from governments' adoption of their approach to economic management.